(1) Field of the Invention
The present invention relates to an apparatus for controlling fuel injection for a direct-injection gasoline engine and a method thereof. More specifically, the invention relates to technology for controlling a fuel injection amount in a mode of injecting fuel in an intake stroke and in a compression stroke in a cycle.
(2) Related Art of the Invention
There has heretofore been known a direct-injection gasoline engine equipped with fuel injection valves for directly injecting fuel into cylinders, which changes over, depending upon operation conditions, a stratified mode for injecting the fuel in a compression stroke only, a homogeneous mode for injecting the fuel in an intake stroke only, and a twice-injection mode for injecting the fuel in the intake stroke and in the compression stroke in a cycle (see Japanese Unexamined Patent Publication No. 5-71383).
In the twice-injection mode, a dividing ratio of a fuel injection amount is set in advance based upon a target average air-fuel ratio, the fuel injection amount in one injection timing is obtained from the total fuel amount to be injected in one cycle and from the dividing ratio, and the remainder of the required amount is injected in the other injection timing.
The twice-injection mode is executed at the time of change-over between the stratified mode and the homogeneous mode in order to avoid a sudden change in output torque of the engine. When the fuel injection amount is changed during each of the injection timings in the twice-injection mode depending upon a change in the target average air-fuel ratio, however, it becomes difficult to set a dividing ratio capable of eliminating a sudden change in the output torque and accomplishing excellent combustion stability.